Iminoxyl radicals and stable products from the one-electron oxidation of 1-methylindole-3-carbaldehyde oximes

The radical intermediates and the stable products formed on one-electron ox idation of 1-methylindole-3-carbaldehyde oxime 2a were compared with those of 1-methylindole-3-carboxamidine oxime 4a in aqueous solution. The dibromi de radical anion generated radiolytically by pulse radiolysis reacted with both 2a and 4a dropC=NOH to yield the radical cations [dropC=NOH](.+), whic h exist in prototropic equilibria with the neutral iminoxyl radicals [>C=NO ](.) (pK(a) = 3.53 +/- 0.03 and 5.01 +/- 0.01 at ionic strength 0.05 M, res pectively). This was confirmed by the observed primary salt-effect which ac celerated the decay of the radical cations but not the iminoxyl radicals. M ethylation of the N-hydroxyimino function in both 2a and 4a precluded depro tonation of the corresponding radical cations [dropC=NOCH3](.+). At low con centrations of 2a and high dose rates the 2a radicals [dropC=NO](.) decayed bimolecularly via unstable dimers to the aldehyde dropC=O, with higher con centrations and lower dose rates favouring the chain-catalysed isomerisatio n of the N-hydroxyimino moiety. Radicals from 4a decay bimolecularly to for m unstable dimers which degrade to produce an amide,, nitrile and carboxyli c acid. The observed differences in-the oxidation chemistry of 2a and 4a pr obably reflect the enhanced stabilisation of iminoxyl radicals through alph a -amino substitution.